1. Field of the Invention
The present invention relates generally to a torque converter used for an automatic transmission or the like. More particularly, it pertains to a lockup control system which properly performs a switching depending upon operation of an accelerator pedal, between a lockup mode in which input and output elements of the torque converter are directly coupled to each other, and a converter mode in which the direct coupling between the input and output elements is released.
2. Description of Related Art
Generally, automatic transmission inclusive of continuously variable transmission comprises a torque converter on its input side, to achieve torque multiplication or absorb torque fluctuation. As generally known in the art, ordinary torque converter suffers from a low power transmission efficiency because power is transmitted from the input element to the output element via a working fluid in the converter. Thus, it is a recent trend to employ a lockup-type torque converter comprising a lockup clutch which is capable of directly coupling the input and output elements when torque multiplication function and/or torque fluctuation absorption function are not required, thereby to improve the fuel economy.
As another measure for improving the fuel economy, there is known a system wherein the engine is provided with a fuel cutting device for cutting the fuel supply to the engine in a coasting state of the vehicle with the accelerator pedal released, as it is then generally unnecessary to supply fuel to the engine. The effectiveness of improvement in the fuel economy by means of the fuel cutting device may be considered proportional to the fuel cutting time in which the fuel supply is being cut. Thus, in order to prolong the fuel cutting time and thereby enhance the fuel economy, it is further known to switch the operating range of the torque converter into the lockup region in the coasting state of the vehicle, i.e., into a so-called coasting lockup region.
When the accelerator pedal is redepressed and the operating range of the torque converter is shifted out of the coasting lockup region, it becomes necessary to disengage the lockup clutch and thereby release the lockup. In this instance, as disclosed in Japanese Patent Application Laid-Open Publication No. 3-96,757, for example, it has been a conventional practice to completely disengage the lockup clutch immediately upon redepression of the accelerator pedal, so as to switch the torque converter from the lockup mode into the converter mode.
The above-mentioned conventional lockup control system capable of switching the operating range of the torque converter into the lockup region in the coasting state of the vehicle proved to be highly advantageous in prolonging the fuel cutting time and thereby enhancing the fuel economy, though it would be still desirous to further refine the technology by improving the control function and feel.